Method of producing noble metal-supported powder, noble metal-supported powder and exhaust gas purifying catalyst

ABSTRACT

In the present invention, slurry is formed by mixing noble metal-supported powder particles ( 3 ) and a binder ( 4 ) with each other in a liquid (Step S 1 ), and the noble metal-supported powder particles ( 3 ) are dispersed by applying vibrations to the slurry (Step S 2 ), and thereafter, the slurry is spray dried while keeping a state where the noble metal-supported powder particles ( 3 ) are dispersed (Step S 3 ), whereby noble metal-supported powder ( 1 ) is produced. In the noble metal-supported powder ( 1 ) produced by such a method, pores through which exhaust gas flows are formed appropriately, and accordingly, exhaust gas purification performance can be enhanced.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.12/740,445, filed Apr. 29, 2010, now issued as U.S. Pat. No. 8,324,127,which is a National Stage of Application No. PCT/JP2008/070336 filed onNov. 7, 2008, which is based upon and claims the benefit of priorityfrom the prior Japanese Application No. 2007-290743, filed on Nov. 8,2007 and Japanese Application No. 2008-283480, Filed on Nov. 4, 2008;the entire contents of all of which are incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to a method of producing noblemetal-supported powder, the noble metal-supported powder and an exhaustgas purifying catalyst.

BACKGROUND ART

An exhaust gas purifying catalyst has been being widely used in order toremove harmful substances such as a hydrocarbon (HC)-based compound,carbon monoxide (CO) and nitrogen oxide (NO_(x)), which are contained inexhaust gas discharged from an internal combustion engine.

The exhaust gas purifying catalyst is a catalyst composed of onecatalyst layer or by stacking two or more catalyst layers on oneanother. Each of the catalyst layers is formed in such a manner thatpowder in which particles of noble metal such as platinum (Pt) aresupported on a support containing, as a main component, a porousinorganic material such as alumina (Al₂O₃) is coated on an inner wallsurface of a refractory inorganic substrate made of ceramics or metalfoil. The refractory inorganic substrate has various shapes as well as amonolithic shape. Such noble metal-supported powder coated on therefractory inorganic substrate is particles with a mean particlediameter approximately ranging from 1 [μm] to 20 [μm]. Independently ofgas diffusibility, the noble metal-supported powder dispersivelysupports, on a surface thereof and in an inside thereof, the noble metalserving as an activity center to purify a harmful gas component in theexhaust gas.

With regard to the exhaust gas purifying catalyst as described above,there is one in which, in a coating layer (catalyst layer), pores with apore diameter of 0.1 to 20 [μm] occupy 60% or more of the entire porevolume, and pores with a pore diameter of 10 to 20 [μm] occupy 20% ormore of the entire pore volume (Patent Citation 1).

-   Patent Citation 1: Japanese Patent Unexamined Publication No.    2002-191988

DISCLOSURE OF INVENTION

The powder that supports the noble metal thereon is composed of primaryparticles in which particles of the noble metal are supported on porousinorganic powder particles, or of secondary particles formed byaggregating the primary particles, and has a mean particle diameterapproximately ranging from 1 to 20 [μm]. In such noble metal-supportedpowder, the noble metal particles which have a function to purify theexhaust gas are present in the inside of the noble metal-supportedpowder and on the surface thereof. In the inside of the noblemetal-supported powder, the pores effective in diffusion of the gas areextremely few, and accordingly, it is difficult for the exhaust gas toreach the noble metal in the inside of the noble metal-supported powderthrough the pores. Hence, among the noble metal present in the noblemetal-supported powder, the noble metal in the inside of the powder hasnot received the harmful gas component efficiently, and has notfunctioned effectively to purify the exhaust gas.

If it is possible to form the pores effective in the gas diffusion inthe inside of the noble metal-supported powder, then the noble metal inthe inside of the powder is enabled to function effectively to purifythe exhaust gas. However, heretofore, there has been no method offorming the pores effective in the gas diffusion in the inside of thenoble metal-supported powder.

Further, if a compound support that supports the noble metal thereon ismixed as particles with a mean particle diameter of 1 [μm] or less intoslurry containing a binder, then powder of the compound support isreaggregated. Accordingly, it has been difficult to fix pieces of thesupport in a state of spacing the pieces apart from one another.

In order to solve the above-described problems, a method of producingnoble metal-supported powder, which is an aspect of the presentinvention, is summarized to include: preparing a slurry by mixing noblemetal-supported powder particles and a binder with each other in aliquid; and dispersing the noble metal-supported powder particles byapplying vibrations to the slurry, and thereafter, spray drying theslurry while keeping a state where the noble metal-supported powderparticles are dispersed.

Moreover, a noble metal-supported powder that is another aspect of thepresent invention is summarized to include: a plurality of noblemetal-supported powder particles; and a binder, wherein a mean particlediameter of the plurality of noble metal-supported powder particles is 1[μm] or less.

Furthermore, an exhaust gas purifying catalyst that is another aspect ofthe present invention is summarized to include: at least one catalystlayer formed on a refractory inorganic substrate by coating thereon aslurry containing the noble metal-supported powder produced by theabove-described method according to the present invention.

Still further, an exhaust gas purifying catalyst that is another aspectof the present invention is summarized to include: at least one catalystlayer on a refractory inorganic substrate, wherein the above-describednoble metal-supported powder according to the present invention iscontained in the catalyst layer.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flowchart explaining an example of a method of producingnoble metal-supported powder according to the present invention.

FIG. 2 is a schematic view of noble metal-supported powder according toan embodiment of the present invention.

FIG. 3 is a graph showing a relationship between an elapsed time afterdispersion of dispersed slurry and degree of dispersion of noblemetal-supported powder particles.

FIG. 4 is a schematic view of an example of a production apparatus foruse in a method of producing the noble metal-supported powder accordingto this embodiment.

FIG. 5 is a schematic view of conventional noble metal-supported powder.

FIG. 6 is a graph showing relationships between pore diameters and flowrates of various gases in Knudsen diffusion.

FIG. 7 is a graph showing a relationship between each drying time of thenoble metal-supported powder per piece of the liquid droplets and eachvolume ratio of the pores thereof effective in gas diffusion.

FIG. 8 is a graph showing a relationship between each volume ratio ofthe pores effective in the gas diffusion in the noble metal-supportedpowder and each HC conversion rate of an exhaust gas purifying catalystafter being subjected to a durability test.

FIG. 9 is a graph showing a relationship between degree of dispersion ofthe slurry and each Pt crystallite diameter of the exhaust gas purifyingcatalyst after being subjected to the durability test.

FIG. 10 is a graph showing a relationship between the degree ofdispersion of the slurry and each HC conversion rate of the exhaust gaspurifying catalyst after being subjected to the durability test.

FIG. 11 is a graph showing a relationship between each Pt crystallitediameter of the exhaust gas purifying catalyst after being subjected tothe durability test and each HC conversion rate of the exhaust gaspurifying catalyst after the durability test.

FIG. 12 is a graph showing a relationship between each particle diameterof a binder mixed with the noble metal-supported powder particles andeach Pt crystallite diameter after the exhaust gas purifying catalyst issubjected to the durability test.

FIG. 13 is a graph showing a relationship each binder amount of theslurry and each Pt crystallite diameter after the durability test.

FIG. 14 is a graph obtained by comparison among Example 1, Example 10and Comparative example 9, showing a relationship between each particlediameter of Pt-supported powder particles and each volume ratio of thepores effective in the gas diffusion.

FIG. 15 is a graph obtained by comparison between Example 1 andComparative example 1, showing a relationship between each particlediameter of the Pt-supported powder particles and each volume ratio ofthe pores effective in the gas diffusion.

BEST MODE FOR CARRYING OUT THE INVENTION

A description will be made below of a method of producing noblemetal-supported powder, the noble metal-supported powder and an exhaustgas purifying catalyst, which are according to the present invention.

FIG. 1 is a flowchart explaining an example of the method of producingnoble metal-supported powder according to the present invention. Asshown in FIG. 1, in order to produce noble metal-supported powder 1 bythe method of producing noble metal-supported powder according to theembodiment of the present invention, first, noble metal-supported powderparticles 3 and a binder 4 are prepared. As noble metal, there can beused one type or two or more types of noble metals such as platinum(Pt), rhodium (Rh) and palladium (Pd), which have a catalytic functionto purify exhaust gas. Moreover, also as a support that supports suchnoble metal thereon, one type or two or more types of porous inorganicmaterials such as alumina can be used. Furthermore, a method forsupporting the noble metal on the support is not particularly limited,either, and for example, an impregnation method and the like can beused.

As the binder, for example, the one containing alumina as a maincomponent can be used.

Next, the noble metal-supported powder particles 3 and the binder 4 aremixed with each other in a liquid, whereby slurry is formed (Step S1).While at least one type of the noble metal-supported powder particles 3is necessary, two or more types of the noble metal-supported powderparticles 3 different in noble metal and/or noble metal-supported powdermay be provided. Note that, in this slurry, there may be contained oxideof at least one type of metal selected from Mn, Fe, Co, Ni, Y, Ba, Zr,La, Ce, Pr and Nd besides the noble metal-supported powder particles 3and the binder 4.

Subsequently, vibrations are applied to the slurry, and the noblemetal-supported powder particles 3 in the slurry are dispersed (StepS2). Next, while keeping a state where the noble metal-supported powderparticles 3 are dispersed in the slurry, the slurry was spray dried(Step S3). In such a way, the noble metal-supported powder 1 includingthe noble metal-supported powder particles 3 and the binder 4 isproduced.

In accordance with the method of producing the noble metal-supportedpowder according to the embodiment of the present invention, the slurryis spray dried (Step S3), and is thereby dried quickly, whereby poreseffective in gas diffusion can be formed in an inside of the noblemetal-supported powder 1. By the formation of the pores, the noble metalin the inside of the powder can also be used effectively to purify theexhaust gas, and accordingly, an exhaust gas purifying catalyst in whichpurification performance is enhanced can be produced.

Moreover, such high dispersion treatment (Step S2) is performedimmediately before the slurry is dried quickly by being spray dried(Step S3), whereby the noble metal-supported powder particles 3 can befixed in a state of being spaced apart from one another. Therefore, anexhaust gas purifying catalyst in which catalyst performance is lessdeteriorated even in use at a high temperature for a long time can beproduced.

Then, the high dispersion treatment and quick dry of the slurry, whichare mentioned above, are combined with each other, whereby noblemetal-supported powder according to the embodiment of the presentinvention, which is as shown in FIG. 2, can be obtained.

FIG. 2 is a schematic view of an example of the noble metal-supportedpowder according to the present invention. FIG. 2A shows one piece ofthe noble metal-supported powder, and this noble metal-supported powder1 is a particle with a diameter approximately ranging from 1 to 20 [μm].In the noble metal-supported powder 1, plural pores 2 which communicatewith the inside thereof are formed. The pores 2 have a pore diameter,for example, approximately ranging from 30 to 100 [nm]. The noblemetal-supported powder 1 is composed of secondary particles formed byaggregating the plural noble metal-supported powder particles 3 with amean particle diameter of 1 [μm] or less. In other words, the noblemetal-supported powder 1 has a structure in which the noblemetal-supported powder particles 3 are aggregated so as to form thepredetermined pores 2.

A schematic view in which the noble metal-supported powder 1 of FIG. 2Ais partially enlarged is shown in FIG. 2B. The noble metal-supportedpowder particles 3 which compose the noble metal-supported powder 1 arefine particles with a mean particle diameter of 1 [μm] or less, and thenoble metal-supported powder particles 3 are bonded to one another bythe binder 4. Noble metal particles 5 are supported on each surface ofthe noble metal-supported powder particles 3. As illustrated, the finenoble metal-supported powder particles 3 are bonded to one another bythe binder 4, whereby the noble metal-supported powder 1 is formed asthe secondary particles, and the noble metal-supported powder 1 has thepores 2 with a pore diameter approximately ranging from 30 to 100 [nm],which communicate with the inside of the noble metal-supported powder 1.

In the method of producing the noble metal-supported powder according tothe present invention, which is for producing the noble metal-supportedpowder as shown in FIG. 2, it is preferable to set degree of dispersionof the noble metal-supported powder particles in the slurry at 33% ormore by applying the vibrations to the slurry. The degree of dispersionis calculated by the following Expression (1) from a measurement resultof dispersion state of the slurry and from the mean particle diameter ofthe particles contained in the slurry. The measurement result ofdispersion state and the mean particle diameter are obtained by using agrain size distribution measuring apparatus.[Degree of dispersion](%)=[Mean particle diameter]/[Dispersion statemeasurement result]×100  (1)

Note that the dispersion state measurement result in Expression (1) is amean particle diameter of the slurry immediately after being subjectedto the high dispersion treatment, the slurry being obtained by mixingthe noble metal-supported powder particles 3 and the binder 4 with eachother, and the dispersion state measurement result is generally obtainedas follows by a grain size distribution measuring apparatus of a dynamiclight scattering mode.

An appropriate amount of the slurry is sampled, which is obtained bymixing the noble metal-supported powder particles 3 and the binder 4with each other and is subjected to the high dispersion treatment by anultrasonic dispersing device. Then, the slurry is filled into atransparence cell. When sizes of the particles in such a mixed statediffer, the particles cause the Brownian movement at different speeds.Accordingly, when a laser beam is irradiated onto the particles from aside surface of the transparent cell, the particles emit scatteringlight corresponding to the sizes thereof. The mean particle diameter ofthe slurry is calculated from a frequency and intensity of thescattering light.

Such vibration application treatment that highly disperses the slurry ina state where the degree of dispersion is 33% or more is performedimmediately before the quick dry of the slurry, whereby the noblemetal-supported powder particles can be fixed in the state of beingspaced apart from one another. Accordingly, movement andaggregation/particle growth of the noble metal among the noblemetal-supported powder particles doe not occur. Therefore, an exhaustgas purifying catalyst can be produced, in which a surface area of thenoble metal is less decreased, and eventually, the catalyst performanceis less deteriorated.

FIG. 3 shows, by a graph, results of measuring a relationship between anelapsed time after the dispersion of the slurry obtained by dispersingthe noble metal-supported powder particles in the liquid and the degreeof dispersion of the noble metal-supported powder particles in theslurry. As shown in FIG. 3, as the time elapses from immediately afterthe dispersion, the degree of dispersion of the noble metal-supportedpowder particles is being reduced.

Then, the degree of dispersion immediately before the slurry is dried isset at the state particularly as high as 33% or more, whereby, in anideal state, it becomes possible to maintain a structure of the noblemetal-supported powder particles in the state where the noblemetal-supported powder particles are spaced apart from one another, andto fix the noble metal-supported powder particles.

A slurry dispersing device can be used for the vibration applicationperformed in order to turn the slurry into the highly dispersed state.Here, if a high-speed stirrer is used as the slurry dispersing device,then the dispersion treatment is performed in accordance with a batchtreatment mode, and continuous treatment between the dispersiontreatment and the quick dry cannot be performed, and accordingly,production efficiency in this case is low. As opposed to this, if theultrasonic dispersing device is used as the slurry dispersing device,then it becomes possible to perform the continuous treatment between thehigh dispersion treatment and the quick dry, and accordingly, productionefficiency in this case is higher than in the case of using thehigh-speed stirrer. Hence, in order to apply the vibrations in themethod of producing the noble metal-supported powder according to thepresent invention, it is particularly preferable to use the ultrasonicdispersing device.

It is preferable to perform the spray drying (Step S3) under a conditionwhere moisture of liquid droplets with a diameter of 1 [mm] or less isevaporated within 1 second per piece of the liquid droplets. The slurryis turned to the liquid droplets with a diameter of 1 [mm] or less, andthe moisture thereof is evaporated within 1 second per piece of theliquid droplets, whereby many pores effective in the gas diffusion canbe formed. By the formation of the pores, the noble metal in the insideof the powder can also be used effectively to purify the exhaust gas,and accordingly, the exhaust gas purifying catalyst in which thepurification performance is enhanced can be produced.

It is preferable to use, as the binder, the one containing inorganicoxide as a main component, in which a mean particle diameter is within arange from 1/10 to 10 times that of the noble metal-supported powderparticles, and to set a mixing ratio of the noble metal-supported powderparticles and the binder within a range from 10:90 to 90:10. As thebinder to be mixed with the noble metal-supported powder particles,there is used the binder containing the inorganic oxide as the maincomponent, in which the mean particle diameter is within the range from1/10 to 10 times that of the noble metal-supported powder particles,then the mixing ratio of the noble metal-supported powder particles andthe binder is set within the range from 10:90 to 90:10, and the slurryis subjected to the high dispersion treatment immediately before thequick dry thereof. In such a way, the noble metal-supported powderparticles can be fixed in the state of being spaced apart from oneanother. Accordingly, the exhaust gas purifying catalyst in which thecatalyst performance is less deteriorated even in use at a hightemperature for a long time can be produced.

As materials of the binder, for example, there are alumina (Al₂O₃),silica (SiO₂), titania (TiO₂), zirconia (ZrO₂), and the like.

As mentioned above, in the slurry obtained by mixing the noblemetal-supported powder particles and the binder with each other, theremay be contained the oxide of at least one type of the metal selectedfrom Mn, Fe, Co, Ni, Y, Ba, Zr, La, Ce, Pr and Nd. Among them, it ispreferable to further add Ce-containing oxide powder particles. TheCe-containing oxide functions as a promoter, and has an oxygenstorage/release capability. Hence, in the exhaust gas purifying catalystproduced of the noble metal-supported powder particles containing theCe-containing oxide, the exhaust gas purification performance isenhanced. Note that, on this Ce-containing oxide powder, there may besupported: the oxide of at least one type of the metal selected from Mn,Fe, Co, Ni, Y, Ba, Zr, La, Pr and Nd; and/or at least one type of noblemetal selected from Au, Pt, Ir, Os, Ag, Pd, Rh and Ru.

Next, a description will be made of the noble metal-supported powderaccording to the present invention. As already mentioned by using FIG.2A, the noble metal-supported powder 1 has the pores 2 which communicatewith the inside thereof, and accordingly, the exhaust gas to be purifiedcan sufficiently reach the inside of the noble metal-supported powder 1.Hence, the exhaust gas is purified also by the noble metal particles 5supported on the noble metal-supported powder particles 3 present in theinside of the noble metal-supported powder 1. Therefore, the exhaust gaspurification performance is enhanced.

The noble metal-supported powder 1 according to the present inventioncan be produced by the above-mentioned method of producing the noblemetal-supported powder according to the present invention.

In the noble metal-supported powder 1, it is preferable that the meanparticle diameter of the noble metal-supported powder particles be 1[μm] or less. By the fact that the mean particle diameter of the noblemetal-supported powder particles is 1 [μm] or less, the pores effectivein the gas diffusion can be ensured in the inside of the noblemetal-supported powder. More preferably, the mean particle diameter ofthe noble metal-supported powder particles is 90 [nm] or more to 550[nm] or less. In both of the case where the mean particle diameter ofthe noble metal-supported powder particles does not reach 90 [nm] andthe case where the mean particle diameter exceeds 550 [nm], it isdifficult to sufficiently ensure the pores effective in the gasdiffusion. By setting the noble metal-supported powder particles at sucha mean particle diameter of 90 [nm] or more to 550 [nm] or less, manypores effective in the gas diffusion can be formed in the inside of thenoble metal-supported powder. By the formation of the pores, the noblemetal in the inside of the powder can also be used effectively to purifythe exhaust gas, and accordingly, the exhaust gas purifying catalyst inwhich the purification performance is enhanced can be produced.

A wet pulverizer using a pulverization medium can be used for finelygranulating the noble metal-supported powder particles; however, amethod of such fine granulation is not limited to a pulverizationprocess.

It is preferable that the noble metal-supported powder 1 be the one, inwhich the binder contains the inorganic oxide as the main component, andthe mean particle diameter is within the range from 1/10 to 10 timesthat of the noble metal-supported powder particles, and that the mixingratio of the noble metal-supported powder particles and the binder bewithin the range from 10:90 to 90:10.

The noble metal-supported powder 1 according to the present inventioncan further contain the Ce-containing oxide powder particles. By thefact that the noble metal-supported powder 1 further includes theCe-containing oxide powder particles, the purification characteristicsof the exhaust gas purifying catalysts are enhanced.

A schematic view of an example of a production apparatus for use in themethod of producing the noble metal-supported powder according to thepresent invention is shown in FIG. 4. The production apparatus of thenoble metal-supported powder includes: a slurry dispersing device 50that applies the vibrations to the slurry and performs the highdispersion treatment therefor; and a rapid dryer 60 that spray dries theslurry highly dispersed by the slurry dispersing device 50.

In FIG. 4, slurry 11 of the noble metal-supported powder particles ishoused in a container 10 for the noble metal-supported powder, andslurry 21 of the binder is housed in a container 20 for the binder. Thenoble metal-supported powder in such noble metal-supported powderparticle slurry 11 is the one in which the mean particle diameterbecomes 1 [μm] or less by being subjected to the step of supporting thenoble metal thereon and the step of being finely granulated. Moreover,the binder in such binder slurry 21 is, as mentioned above, the onehaving the particle diameter corresponding to the particle diameter ofthe noble metal-supported powder particles in the noble metal-supportedpowder particle slurry 11. Such a noble metal-supported powder container10 and such a binder container 20 individually include stirring means,and the stirring means suppress aggregation of the noble metal-supportedpowder particle slurry 11 and the binder slurry 21, each of which ishoused in the container.

The noble metal-supported powder particle slurry 11 is supplied to amixer 40 by a pump 30, the binder slurry 21 is also supplied to themixer 40 by a pump 30 in a similar way, and the slurry obtained bymixing the noble metal-supported powder particles and the binder witheach other is formed by the mixer 40. The mixing ratio of the noblemetal-supported powder particle slurry 11 and the binder slurry 21 isadjusted so as to become the above-mentioned appropriate range.

The noble metal-supported powder particle slurry 11 and the binderslurry 21, which are mixed with each other by the mixer 40, are guidedto the slurry dispersing device 50. The slurry dispersing device 50 ispreferably the ultrasonic dispersing device shown as a slurry dispersingdevice 50 in FIG. 4. In the slurry dispersing device 50, the vibrationsare applied to the slurry by an ultrasonic vibrator 51 immersed into theslurry, whereby the noble metal-supported powder particles and thebinder in the slurries are uniformly dispersed. The degree of dispersionof the noble metal-supported powder particles in the slurry can be setat 33% or more by the ultrasonic dispersing device.

The slurry in the state of being sufficiently dispersed by the slurrydispersing device 50 is supplied to the rapid dryer 60 through a pump,and is dried by the rapid dryer 60 in a short time. The rapid dryer 60is a device capable of turning the slurry into liquid droplets with adiameter of 1 [mm] or less, and evaporating moisture of the liquiddroplets within 10 [second] per piece of the liquid droplets. Spraydryers are preferable as the rapid dryer 60. Among the spray dryers, aspray dryer shown as the rapid dryer 60 in FIG. 4 is a device thatincludes, in an upper portion of a dryer chamber thereof, an atomizer 61ejecting the slurry guided thereto from the slurry dispersing device 50,and spray dries the slurry in such a manner that a rotary disc 62 on atip end of the atomizer 61 rotates at a high speed to finely granulateand spray the slurry in a predetermined high-temperature atmosphere. Therotary disc atomizer is particularly easy to control the liquid dropletdiameter and the temperature of the dryer chamber, and can obtainuniform powder, and accordingly, is suitable for producing the noblemetal-supported powder according to the present invention. Note that theatomizer may be the one of a nozzle type, which can finely granulate andspray the slurry. The slurry is quickly dried by using the rapid dryer60 as described above, whereby the noble metal-supported powder composedof the noble metal-supported powder particles and the binder can beformed.

Such a combination of the slurry dispersing device 50 as the ultrasonicdispersing device and the rapid dryer 60 as the spray dryer enables thecontinuous treatment between the dispersion treatment and the quick dry,and is a desirable combination for the production apparatus also from aviewpoint that high production efficiency can be obtained.

The slurry containing the noble metal-supported powder according to thepresent invention is coated on a refractory inorganic substrate, and atleast one catalyst layer is formed, whereby the exhaust gas purifyingcatalyst according to the present invention is produced.

The exhaust gas purifying catalyst according to the present invention,which is as described above, is the one in which at least one catalystlayer is formed on the refractory inorganic substrate in such a mannerthat the slurry containing the noble metal-supported powder produced bythe method of producing the noble metal-supported powder according tothe present invention is coated on the refractory inorganic substrate,or the one in which at least one catalyst layer is provided on therefractory inorganic substrate, and the noble metal-supported powderaccording to the present invention is contained in the catalyst layer.

In order to clarify an advantage of the noble metal-supported powderaccording to the embodiment of the present invention, which is shown inFIG. 2, a schematic view of the conventional noble metal-supportedpowder is shown in FIG. 5 for comparison. Noble metal-supported powder100 shown in FIG. 5 is the one in which noble metal particles 300 arepresent at portions, which are poor in gas flow, in an inside of thenoble metal-supported powder 100 with a mean particle diameterapproximately ranging from 1 to 20 [μm], and noble metal particles 500are formed on a surface of the noble metal-supported powder 100. Hence,the exhaust gas to be purified does not sufficiently reach the noblemetal particles 300, and accordingly, the noble metal particles 300 havenot functioned effectively to purify the exhaust gas. As opposed tothis, in the noble metal-supported powder 1 according to the embodimentof the present invention, which is shown in FIG. 2, the pores 2effective in the gas diffusion are formed to the inside thereof, and thenoble metal in the inside of the powder can also be used effectively.This is obvious from a difference in structure between the powderillustrated in FIG. 5 and the powder illustrated in FIG. 2.

EXAMPLES

A description will be specifically made below of the present inventionbased on examples.

Example 1

Pt-supported powder was pulverized so that a mean particle diameterthereof could become 150 [nm], and was turned into Pt-supported powderparticles. Alumina (Al₂O₃) was used as a material of the powder thatsupported Pt thereon. Moreover, a wet pulverizer using a pulverizationmedium was used for pulverizing the Pt-supported powder.

The Pt-supported powder particles and a binder were mixed with eachother in a weight ratio of 50:50, and were turned into slurry with asolid concentration of approximately 10%. Here, as the binder, the onewas used, which contained the alumina as a main component, and had amean particle diameter that was ½ of that of the Pt-supported powderparticles.

Immediately before drying the slurry obtained by mixing the Pt-supportedpowder particles and the binder with each other, degree of dispersion ofthe slurry was set at 91%. Such setting of the degree of dispersion wasperformed in such a manner that an ultrasonic dispersing device wasarranged immediately before a rapid dryer, and the slurry was dispersedwhile adjusting a high dispersion output of the ultrasonic dispersingdevice so that the target degree of dispersion can be obtained, and wassupplied to the rapid dryer within 1 minute after such high dispersiontreatment. Here, for measurement for calculating the degree ofdispersion, a grain size distribution measuring apparatus of the dynamiclight scattering mode was used, and the degree of dispersion wascalculated by the following Expression (1).[Degree of dispersion](%)=[Mean particle diameter]/[Dispersion statemeasurement result]×100  (1)

Next, the slurry, which was obtained by mixing the Pt-supported powderparticles and the binder with each other and was subjected to the highdispersion treatment, was rapidly dried in a liquid droplet state. Fordrying the slurry, a spray dryer as a rapid dryer was used. An inlettemperature of the spray dryer was set at 350° C., and an amount of heatapplied to the slurry and a slurry treatment rate were adjusted so thatan outlet temperature of the spray dryer could become 140° C. Here, amean particle diameter of the liquid droplets was set at approximately10 [μm].

Powder obtained by drying the slurry was subjected to heating treatmentat 550° C. for 3 hours. A muffle furnace was used for the heatingtreatment. In such a way, Pt-supported powder was obtained (powder A).

A pore distribution of the obtained powder was measured, and a volumeratio of pores effective in the gas diffusion, which were present in aparticle inside, was calculated by the following Expression (2).[Volume ratio of pores effective in gas diffusion]=[Volume of pores witha diameter of 30 to 100 nm]/[Volume of powder]  (2)

In Expression (2), the gas diffusion in the pores with a diameter ofless than 100 [nm] is referred to as Knudsen diffusion, and isrepresented by Expression (3).q=(4/3)·rε(2RT/πM)^(1/2)·{(p ₁ −p ₂)/(1·RT)}  (3)where q is a gas flow rate per unit area and unit time, r is a radius ofthe pores, ε is porosity, R is a gas constant, T is an absolutetemperature, M is a molecular weight of the gas, p1 and p2 are pressuresof the gas on both sides of the pores, and l is a length of the pores.

Relationships between the pore diameters and flow rates of various gasesin the Knudsen diffusion are shown by a graph in FIG. 6. From FIG. 6, itis seen that, among the pores of the powder, pores with a diameter of 30to 100 [nm] are effective in the gas diffusion. Therefore, the volumeratio of the pores effective in the gas diffusion was defined as inExpression (2).

In Example 1, the above-described volume ratio of the pores effective inthe gas diffusion was 23.8%.

Example 2

Example 2 is an example of an exhaust gas purifying catalyst using thenoble metal-supported powder of Example 1.

98.2 g of the powder A obtained in Example 1, 13.5 g of alumina sol,further 177.6 g of water, and 10.8 g of a 10% aqueous nitric acidsolution were added to one another to obtain slurry. The slurry thusobtained was pulverized by the wet pulverizer using the pulverizationmedium, and was turned to slurry with a mean particle diameter of 3 [μm](slurry A).

This slurry A was coated on a honeycomb support made of ceramics,followed by drying, and was thereafter baked at 400° C. for 1 hour,whereby a catalyst was formed (catalyst A).

A durability test was performed for this catalyst A. In the durabilitytest, the catalyst A was mounted on an exhaust system of a gasolineengine with a displacement of 3500 cc, an inlet temperature of thecatalyst was set at 900° C., and the gasoline engine was operated for 30hours.

With regard to the catalyst A after being subjected to the durabilitytest, a conversion rate of hydrocarbon when the inlet temperature of thecatalyst was 400° C. was obtained in accordance with the followingexpression in the state where the catalyst was mounted on the exhaustsystem of the gasoline engine with a displacement of 3500 cc.HC conversion rate (%)=[(Catalyst inlet HC concentration)−(Catalystoutlet HC concentration)]/(Catalyst inlet HC concentration)×100

At this time, the HC conversion rate was 44%, which was equal to orlarger than a conversion rate (40%) necessary in this evaluationcondition in order to achieve target performance (Japanese exhaust gasregulation value (reduction level) in 2005: −75%).

With regard to the catalyst A after being subjected to the durabilitytest, powder was sampled from a catalyst coating layer thereof, and acrystallite diameter thereof was calculated from a full width at halfmaximum of an XRD reflection peak of Pt (111). At this time, thecrystallite diameter was 12 [nm].

Example 3

Example 3 is an example of the noble metal-supported powder containingCe and the exhaust gas purifying catalyst using the noblemetal-supported powder.

Pt-supported powder in which a supporting concentration of Pt was settwice that of Example 1 was pulverized so that a mean particle diameterthereof could become 150 [nm], and was turned into Pt-supported powderparticles. Alumina (Al₂O₃) was used as a material of the powder thatsupported Pt thereon. Moreover, the wet pulverizer using thepulverization medium was used for pulverizing the Pt-supported powder.

Moreover, oxide powder containing Ce was pulverized so that a meanparticle diameter thereof could become 150 [nm], and was turned intoCe-containing oxide powder particles. The wet pulverizer using thepulverization medium was used for pulverizing the Ce-containing oxidepowder. Note that, on this Ce-containing oxide powder, there may besupported: the oxide of at least one type of the metal selected from Mn,Fe, Co, Ni, Y, Ba, Zr, La, Pr and Nd; and/or the at least one type ofnoble metal selected from Au, Pt, Ir, Os, Ag, Pd, Rh and Ru.

The above-described Pt-supported powder particles, the above-describedCe-containing oxide powder particles and a binder were mixed with oneanother in a weight ratio of 25:25:50, and were turned into slurry witha solid concentration of approximately 10%. Here, as the binder, the onewas used, which contained the alumina as a main component, and had amean particle diameter that was 10 times that of the Pt-supported powderparticles.

Immediately before drying the slurry obtained by mixing the Pt-supportedpowder particles, the Ce-containing oxide powder particles and thebinder with one another, degree of dispersion of the slurry was set at91%. Such setting of the degree of dispersion was performed in such amanner that the ultrasonic dispersing device was arranged immediatelybefore the rapid dryer, the high dispersion output thereof was adjustedso as to obtain the target degree of dispersion, and the slurry wassupplied to the rapid dryer within 1 minute after the high dispersiontreatment. Here, for the measurement for calculating the degree ofdispersion, the grain size distribution measuring apparatus of thedynamic light scattering mode was used, and the degree of dispersion wascalculated by the above-mentioned Expression (1).

A process from drying of the slurry subjected to the high dispersiontreatment to heating treatment thereof is the same as that of Example 1.In such a way, Pt-supported powder was obtained (powder B).

A pore distribution of the obtained powder was measured, and a volumeratio of the pores effective in the gas diffusion, which were present inthe particle inside, was calculated by the above-mentioned Expression(2). At this time, the volume ratio of the pores effective in the gasdiffusion was 23.8%.

A process from preparation of slurry (slurry B) containing the powder Bthrough coating, drying and baking thereof to formation of a catalyst(catalyst B) is the same as that of Example 2.

A durability test was performed for this catalyst B. In the durabilitytest, the catalyst B was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, an inlet temperature of thecatalyst was set at 900° C., and the gasoline engine was operated for 30hours.

With regard to the catalyst B after being subjected to the durabilitytest, the catalyst was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, and a conversion rate ofhydrocarbon when the inlet temperature of the catalyst was 400° C. wasobtained in accordance with the following expression.HC conversion rate (%)=[(Catalyst inlet HC concentration)−(Catalystoutlet HC concentration)]/(Catalyst inlet HC concentration)×100

At this time, the HC conversion rate was 46%, which was equal to orlarger than the conversion rate (40%) necessary in this evaluationcondition in order to achieve the target performance (Japanese exhaustgas regulation value (reduction level) in 2005: −75%).

Example 4

Example 4 is an example in which degree of dispersion of slurry isdifferent from that of Example 4.

A process to preparation of mixed slurry of Pt-supported powderparticles and a binder is the same as that of Example 1.

Immediately before drying the slurry obtained by mixing the Pt-supportedpowder particles and the binder with each other, the degree ofdispersion of the slurry was set at 50%. Such setting of the degree ofdispersion was performed in such a manner that the ultrasonic dispersingdevice was arranged immediately before the rapid dryer, the highdispersion output thereof was adjusted so as to obtain the target degreeof dispersion, and the slurry was supplied to the rapid dryer within 1minute after the high dispersion treatment. Here, for the measurementfor calculating the degree of dispersion, the grain size distributionmeasuring apparatus of the dynamic light scattering mode was used, andthe degree of dispersion was calculated by the above-mentionedExpression (1).

A process from drying of the slurry subjected to the high dispersiontreatment to heating treatment thereof is the same as that of Example 1.In such a way, Pt-supported powder was obtained (powder C).

A process from preparation of slurry (slurry C) containing the powder Cthrough coating, drying and baking thereof to formation of a catalyst(catalyst C) is the same as that of Example 2.

A durability test was performed for this catalyst C. In the durabilitytest, the catalyst C was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, an inlet temperature of thecatalyst was set at 900° C., and the gasoline engine was operated for 30hours.

With regard to the catalyst C after being subjected to the durabilitytest, the catalyst was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, and a conversion rate ofhydrocarbon when the inlet temperature of the catalyst was 400° C. wasobtained in accordance with the following expression.HC conversion rate (%)=[(Catalyst inlet HC concentration)−(Catalystoutlet HC concentration)]/(Catalyst inlet HC concentration)×100

At this time, the HC conversion rate was 42%, which was equal to orlarger than the conversion rate (40%) necessary in this evaluationcondition in order to achieve the target performance (Japanese exhaustgas regulation value (reduction level) in 2005: −75%).

Example 5

Example 5 is an example in which degree of dispersion of slurry isdifferent from those of Examples 1 and 2.

A process to preparation of mixed slurry of Pt-supported powderparticles and a binder is the same as that of Example 1.

Immediately before drying the slurry obtained by mixing the Pt-supportedpowder particles and the binder with each other, the degree ofdispersion of the slurry was set at 33%. Such setting of the degree ofdispersion was performed in such a manner that the high-speed stirrerwas arranged immediately before the rapid dryer, the number ofrevolutions of a stirring impeller thereof was adjusted so as to obtainthe target degree of dispersion, and the slurry was supplied to therapid dryer within 1 minute after the high-speed stirring treatment.Here, for the measurement for calculating the degree of dispersion, thegrain size distribution measuring apparatus of the dynamic lightscattering mode was used, and the degree of dispersion was calculated bythe above-mentioned Expression (1).

A process from drying of the slurry subjected to the high-speed stirringtreatment to heating treatment thereof is the same as that of Example 1.In such a way, Pt-supported powder was obtained (powder D).

A process from preparation of slurry (slurry D) containing the powder Dthrough coating, drying and baking thereof to formation of a catalyst(catalyst D) is the same as that of Example 2.

A durability test was performed for this catalyst D. In the durabilitytest, the catalyst D was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, an inlet temperature of thecatalyst was set at 900° C., and the gasoline engine was operated for 30hours.

With regard to the catalyst D after being subjected to the durabilitytest, the catalyst was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, and a conversion rate ofhydrocarbon when the inlet temperature of the catalyst was 400° C. wasobtained in accordance with the following expression.HC conversion rate (%)=[(Catalyst inlet HC concentration)−(Catalystoutlet HC concentration)]/(Catalyst inlet HC concentration)×100

At this time, the HC conversion rate was 40%, which was equal to orlarger than the conversion rate (40%) necessary in this evaluationcondition in order to achieve the target performance (Japanese exhaustgas regulation value (reduction level) in 2005: −75%).

With regard to the catalyst D after being subjected to the durabilitytest, powder was sampled from a catalyst coating layer thereof, and acrystallite diameter thereof was calculated from a full width at halfmaximum of the XRD reflection peak of Pt (111). At this time, thecrystallite diameter was 15 [nm].

Example 6

Example 6 is an example in which a drying time of slurry is differentfrom that of Example 1.

A process to preparation of mixed slurry of Pt-supported powderparticles and a binder is the same as that of Example 1.

Immediately before drying the slurry obtained by mixing the Pt-supportedpowder particles and the binder with each other, the degree ofdispersion of the slurry was set at 91%. Such setting of the degree ofdispersion was performed in such a manner that the ultrasonic dispersingdevice was arranged immediately before the rapid dryer, the highdispersion output thereof was adjusted so as to obtain the target degreeof dispersion, and the slurry was supplied to the rapid dryer within 1minute after the high dispersion treatment. Here, for the measurementfor calculating the degree of dispersion, the grain size distributionmeasuring apparatus of the dynamic light scattering mode was used, andthe degree of dispersion was calculated by the above-mentionedExpression (1).

Next, the slurry, which was obtained by mixing the Pt-supported powderparticles and the hinder with each other and was subjected to the highdispersion treatment, was dried within 1 second per piece of the liquiddroplets. For drying the slurry, the spray dryer as the rapid dryer wasused. The inlet temperature of the spray dryer was set at 200° C., andthe amount of heat applied to the slurry and the slurry treatment ratewere adjusted so that the outlet temperature of the spray dryer couldbecome 70° C. Here, the mean particle diameter of the liquid dropletswas set at approximately 120 [μm].

Powder obtained by drying the slurry was subjected to heating treatmentat 550° C. for 3 hours. The muffle furnace was used for the heatingtreatment. In such a way, Pt-supported powder was obtained (powder E).

A pore distribution of the obtained powder was measured, and a volumeratio of the pores effective in the gas diffusion, which were present inthe particle inside, was calculated by the above-mentioned Expression(2). At this time, the volume ratio of the pores effective in the gasdiffusion was 22.0%.

Example 7

Example 7 is an example of an exhaust gas purifying catalyst using thenoble metal-supported powder of Example 6.

A process from preparation of slurry (slurry E) containing the powder Ethrough coating, drying and baking thereof to formation of a catalyst(catalyst E) is the same as that of Example 2.

A durability test was performed for this catalyst E. In the durabilitytest, the catalyst D was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, an inlet temperature of thecatalyst was set at 900° C., and the gasoline engine was operated for 30hours.

With regard to the catalyst E after being subjected to the durabilitytest, the catalyst was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, and a conversion rate ofhydrocarbon when the inlet temperature of the catalyst was 400° C. wasobtained in accordance with the following expression.HC conversion rate (%)=[(Catalyst inlet HC concentration)−(Catalystoutlet HC concentration)]/(Catalyst inlet HC concentration)×100

At this time, the HC conversion rate was 40%, which was equal to orlarger than the conversion rate (40%) necessary in this evaluationcondition in order to achieve the target performance (Japanese exhaustgas regulation value (reduction level) in 2005: −75%).

Example 8

Example 8 is an example in which a particle diameter of a binder isdifferent from those of Examples 1 and 2.

Pt-supported powder was pulverized so that a mean particle diameterthereof could become 150 [nm], and was turned into Pt-supported powderparticles. Alumina (Al₂O₃) was used as a material of the powder thatsupported Pt thereon. Moreover, the wet pulverizer using thepulverization medium was used for pulverizing the Pt-supported powder.

The Pt-supported powder particles and a binder were mixed with eachother in a weight ratio of 50:50, and were turned into slurry with asolid concentration of approximately 10%. Here, as the binder, the onewas used, which contained the alumina as a main component, and had amean particle diameter that was 10 times that of the Pt-supported powderparticles.

Immediately before drying the slurry obtained by mixing the Pt-supportedpowder particles and the binder with each other, degree of dispersion ofthe slurry was set at 91%. Such setting of the degree of dispersion wasperformed in such a manner that the ultrasonic dispersing device wasarranged immediately before the rapid dryer, the high dispersion outputthereof was adjusted so as to obtain the target degree of dispersion,and the slurry was supplied to the rapid dryer within 1 minute after thehigh dispersion treatment. Here, for the measurement for calculating thedegree of dispersion, the grain size distribution measuring apparatus ofthe dynamic light scattering mode was used, and the degree of dispersionwas calculated by the above-mentioned Expression (1).

A process from drying of the slurry subjected to the high dispersiontreatment to heating treatment thereof is the same as that of Example 1.In such a way, Pt-supported powder was obtained (powder F).

A process from preparation of slurry (slurry F) containing the powder Fthrough coating, drying and baking thereof to formation of a catalyst(catalyst F) is the same as that of Example 2.

A durability test was performed for this catalyst F. In the durabilitytest, the catalyst F was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, an inlet temperature of thecatalyst was set at 900° C., and the gasoline engine was operated for 30hours.

With regard to the catalyst F after being subjected to the durabilitytest, powder was sampled from a catalyst coating layer thereof, and acrystallite diameter thereof was calculated from a full width at halfmaximum of the XRD reflection peak of Pt (111). At this time, thecrystallite diameter was 15 [nm].

Example 9

Example 9 is an example in which a mixing ratio of noble metal-supportedpowder particles and a binder is different from those of Examples 1 and2.

Pt-supported powder was pulverized so that a mean particle diameterthereof could become 150 [nm], and was turned into Pt-supported powderparticles. Alumina (Al₂O₃) was used as a material of the powder thatsupported Pt thereon. Moreover, the wet pulverizer using thepulverization medium was used for pulverizing the Pt-supported powder.

The Pt-supported powder particles and a binder were mixed with eachother in a weight ratio of 90:10, and were turned into slurry with asolid concentration of approximately 10%. Here, as the binder, the onewas used, which contained the alumina as a main component, and had amean particle diameter that was ½ of that of the Pt-supported powderparticles.

Immediately before drying the slurry obtained by mixing the Pt-supportedpowder particles and the binder with each other, degree of dispersion ofthe slurry was set at 91%. Such setting of the degree of dispersion wasperformed in such a manner that the ultrasonic dispersing device wasarranged immediately before the rapid dryer, the high dispersion outputthereof was adjusted so as to obtain the target degree of dispersion,and the slurry was supplied to the rapid dryer within 1 minute after thehigh dispersion treatment. Here, for the measurement for calculating thedegree of dispersion, the grain size distribution measuring apparatus ofthe dynamic light scattering mode was used, and the degree of dispersionwas calculated by the above-mentioned Expression (1).

A process from drying of the slurry subjected to the high dispersiontreatment to heating treatment thereof is the same as that of Example 1.In such a way, Pt-supported powder was obtained (powder G).

A process from preparation of slurry (slurry G) containing the powder Gthrough coating, drying and baking thereof to formation of a catalyst(catalyst G) is the same as that of Example 2.

A durability test was performed for this catalyst G. In the durabilitytest, the catalyst G was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, an inlet temperature of thecatalyst was set at 900° C., and the gasoline engine was operated for 30hours.

With regard to the catalyst G after being subjected to the durabilitytest, powder was sampled from a catalyst coating layer thereof, and acrystallite diameter thereof was calculated from a full width at halfmaximum of the XRD reflection peak of Pt (111). At this time, thecrystallite diameter was 15 [nm].

Example 10

Example 10 is an example in which a particle diameter of noblemetal-supported powder particles is different from those of Examples 1and 2.

Pt-supported powder was pulverized so that a mean particle diameterthereof could become 105 [nm], and was turned into Pt-supported powderparticles. Alumina (Al₂O₃) was used as a material of the powder thatsupported Pt thereon. Moreover, the wet pulverizer using thepulverization medium was used for pulverizing the Pt-supported powder.

The Pt-supported powder particles and a binder were mixed with eachother in a weight ratio of 50:50, and were turned into slurry with asolid concentration of approximately 10%. Here, as the binder, the onewas used, which contained the alumina as a main component, and had amean particle diameter that was ½ of that of the Pt-supported powderparticles.

A process from high dispersion treatment of slurry obtained by mixingthe Pt-supported powder particles and the binder with each other toheating treatment thereof is the same as that of Example 1.

In such a way, Pt-supported powder was obtained (powder O).

A pore distribution of the obtained powder was measured, and a volumeratio of the pores effective in the gas diffusion, which were present inthe particle inside, was calculated by the above-mentioned Expression(2). At this time, the volume ratio of the pores effective in the gasdiffusion was 22.3%.

Comparative Example 1

Comparative example 1 is an example in which the mean particle diameterof the noble metal-supported powder particles is large.

Pt-supported powder in which a supporting concentration of Pt was set ata half of that of Example 1 was pulverized so that a mean particlediameter thereof could become 3 [μm]. A dry pulverizer using apulverization medium was used for pulverizing the Pt-supported powder.In such a way, Pt-supported powder was obtained (powder H).

A pore distribution of the obtained powder was measured, and a volumeratio of the pores effective in the gas diffusion, which were present inthe particle inside, was calculated by the above-mentioned Expression(2). At this time, the volume ratio of the pores effective in the gasdiffusion was 10.8%.

Comparative Example 2

Comparative example 2 is an example of an exhaust gas purifying catalystusing the noble metal-supported powder of Comparative example 1.

A process from preparation of slurry (slurry H) containing the powder Hthrough coating, drying and baking thereof to formation of a catalyst(catalyst H) is the same as that of Example 2.

A durability test was performed for this catalyst H. In the durabilitytest, the catalyst H was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, an inlet temperature of thecatalyst was set at 900° C., and the gasoline engine was operated for 30hours.

With regard to the catalyst H after being subjected to the durabilitytest, the catalyst was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, and a conversion rate ofhydrocarbon when the inlet temperature of the catalyst was 400° C. wasobtained in accordance with the following expression.HC conversion rate (%)=[(Catalyst inlet HC concentration)−(Catalystoutlet HC concentration)]/(Catalyst inlet HC concentration)×100

At this time, the HC conversion rate was 5%.

With regard to the catalyst H after being subjected to the durabilitytest, powder was sampled from a catalyst coating layer thereof, and acrystallite diameter thereof was calculated from a full width at halfmaximum of the XRD reflection peak of Pt (111). At this time, thecrystallite diameter was 100 [nm].

Comparative Example 3

Comparative example 3 is an example in which the slurry was notsubjected to the dispersion treatment, but was dried slowly.

A process to preparation of mixed slurry of Pt-supported powderparticles and a binder is the same as that of Example 1.

The slurry obtained by mixing the Pt-supported powder particles and thebinder with each other was not subjected to the dispersion treatment,but was dried at a speed equivalent to approximately 150 sec per pieceof liquid droplets with a mean particle diameter of approximately 1000[μm]. Such drying of the slurry was performed in such a manner that theslurry was dried at reduced pressure in an evaporator, and wasthereafter dried in a dryer of 120° C.

Powder obtained by drying the slurry was subjected to heating treatmentat 550° C. for 3 hours. The muffle furnace was used for the heatingtreatment. In such a way, Pt-supported powder was obtained (powder I).

A pore distribution of the obtained powder was measured, and a volumeratio of the pores effective in the gas diffusion, which were present inthe particle inside, was calculated by the above-mentioned Expression(2). At this time, the volume ratio of the pores effective in the gasdiffusion was 20.3%.

Comparative Example 4

Comparative example 4 is an example of an exhaust gas purifying catalystusing the noble metal-supported powder of Comparative example 3.

A process from preparation of slurry (slurry I) containing powder Ithrough coating, drying and baking thereof to formation of a catalyst(catalyst I) is the same as that of Example 2.

A durability test was performed for this catalyst I. In the durabilitytest, the catalyst I was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, an inlet temperature of thecatalyst was set at 900° C., and the gasoline engine was operated for 30hours.

With regard to the catalyst I after being subjected to the durabilitytest, the catalyst was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, and a conversion rate ofhydrocarbon when the inlet temperature of the catalyst was 400° C. wasobtained in accordance with the following expression.HC conversion rate (%)=[(Catalyst inlet HC concentration)−(Catalystoutlet HC concentration)]/(Catalyst inlet HC concentration)×100

At this time, the HC conversion rate was 10%.

Comparative Example 5

Comparative example 5 is an example in which the degree of dispersion ofthe slurry is small.

A process to preparation of mixed slurry of Pt-supported powderparticles and a binder is the same as that of Example 1.

Immediately before drying the slurry obtained by mixing the Pt-supportedpowder particles and the binder with each other, the degree ofdispersion of the slurry was set at 25%. Such setting of the degree ofdispersion was performed in such a manner that the high-speed stirrerwas arranged immediately before the rapid dryer, the number ofrevolutions of the stirring impeller thereof was adjusted so as toobtain the target degree of dispersion, and the slurry was supplied tothe rapid dryer within 1 minute after the high-speed stirring treatment.Here, for the measurement for calculating the degree of dispersion, thegrain size distribution measuring apparatus of the dynamic lightscattering mode was used, and the degree of dispersion was calculated bythe above-mentioned Expression (1).

A process from drying of the slurry subjected to the high-speed stirringtreatment to heating treatment thereof is the same as that of Example 1.In such a way, Pt-supported powder was obtained (powder J).

A process from preparation of slurry (slurry J) containing the powder Jthrough coating, drying and baking thereof to formation of a catalyst(catalyst J) is the same as that of Example 2.

A durability test was performed for this catalyst J. In the durabilitytest, the catalyst J was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, an inlet temperature of thecatalyst was set at 900° C., and the gasoline engine was operated for 30hours.

With regard to the catalyst J after being subjected to the durabilitytest, the catalyst was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, and a conversion rate ofhydrocarbon when the inlet temperature of the catalyst was 400° C. wasobtained in accordance with the following expression.HC conversion rate (%) [(Catalyst inlet HC concentration)−(Catalystoutlet HC concentration)]/(Catalyst inlet HC concentration)×100

At this time, the HC conversion rate was 30%.

Comparative Example 6

Comparative example 6 is an example in which the degree of dispersion ofthe slurry is small.

A process to preparation of mixed slurry of Pt-supported powderparticles and a binder is the same as that of Example 1.

The slurry obtained by mixing the Pt-supported powder particles and thebinder with each other was not subjected to the dispersion treatment,but was rapidly dried in a state of liquid droplets. The spray dyer asthe rapid dryer was used for drying the slurry, the inlet temperature ofthe spray dryer was set at 350° C., and the amount of heat applied tothe slurry and the slurry treatment rate were adjusted so that theoutlet temperature of the spray dryer could become 140° C. Here, themean particle diameter of the liquid droplets was set at approximately10 [μm].

Note that the degree of dispersion of the slurry, which was measuredimmediately before drying the slurry, was 19%. Here, for the measurementfor calculating the degree of dispersion, the grain size distributionmeasuring apparatus of the dynamic light scattering mode was used, andthe degree of dispersion was calculated by the above-mentionedExpression (1).

Powder obtained by drying the slurry was subjected to heating treatmentat 550° C. for 3 hours. The muffle furnace was used for the heatingtreatment. In such a way, Pt-supported powder was obtained (powder K).

A process from preparation of slurry (slurry K) containing the powder Kthrough coating, drying and baking thereof to formation of a catalyst(catalyst K) is the same as that of Example 2.

A durability test was performed for this catalyst K. In the durabilitytest, the catalyst K was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, an inlet temperature of thecatalyst was set at 900° C., and the gasoline engine was operated for 30hours.

With regard to the catalyst K after being subjected to the durabilitytest, the catalyst was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, and a conversion rate ofhydrocarbon when the inlet temperature of the catalyst was 400° C. wasobtained in accordance with the following expression.HC conversion rate (%)=[(Catalyst inlet HC concentration)−(Catalystoutlet HC concentration)]/(Catalyst inlet HC concentration)×100

At this time, the HC conversion rate was 15%.

With regard to the catalyst K after being subjected to the durabilitytest, powder was sampled from a catalyst coating layer thereof, and acrystallite diameter thereof was calculated from a full width at halfmaximum of the XRD reflection peak of Pt (111). At this time, thecrystallite diameter was 29 [nm].

Comparative Example 7

Comparative example 7 is an example in which the mean particle diameterof the binder is 20 times that of the noble metal-supported powderparticles.

Pt-supported powder was pulverized so that a mean particle diameterthereof could become 150 [nm], and was turned into Pt-supported powderparticles. Alumina (Al₂O₃) was used as a material of the powder thatsupported Pt thereon. Moreover, the wet pulverizer using thepulverization medium was used for pulverizing the Pt-supported powder.

The Pt-supported powder particles and a binder were mixed with eachother in a weight ratio of 50:50, and were turned into slurry with asolid concentration of approximately 10%. Here, as the binder, the onewas used, which contained the alumina as a main component, and had amean particle diameter that was 20 times that of the Pt-supported powderparticles.

Immediately before drying the slurry obtained by mixing the Pt-supportedpowder particles and the binder with each other, degree of dispersion ofthe slurry was set at 91%. Such setting of the degree of dispersion wasperformed in such a manner that the ultrasonic dispersing device wasarranged immediately before the rapid dryer, the high dispersion outputthereof was adjusted so as to obtain the target degree of dispersion,and the slurry was supplied to the rapid dryer within 1 minute after thehigh dispersion treatment. Here, for the measurement for calculating thedegree of dispersion, the grain size distribution measuring apparatus ofthe dynamic light scattering mode was used, and the degree of dispersionwas calculated by the above-mentioned Expression (1).

A process from drying of the slurry subjected to the high dispersiontreatment to heating treatment thereof is the same as that of Example 1.In such a way, Pt-supported powder was obtained (powder L).

A process from preparation of slurry (slurry L) containing the powder Lthrough coating, drying and baking thereof to formation of a catalyst(catalyst L) is the same as that of Example 2.

A durability test was performed for this catalyst L. In the durabilitytest, the catalyst L was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, an inlet temperature of thecatalyst was set at 900° C., and the gasoline engine was operated for 30hours.

With regard to the catalyst L after being subjected to the durabilitytest, powder was sampled from a catalyst coating layer thereof, and acrystallite diameter thereof was calculated from a full width at halfmaximum of the XRD reflection peak of Pt (111). At this time, thecrystallite diameter was 35 [nm].

Comparative Example 8

Comparative example 8 is an example in which an amount of the binder issmaller than that of the noble metal-supported powder.

Pt-supported powder was pulverized so that a mean particle diameterthereof could become 150 [nm], and was turned into Pt-supported powderparticles. Alumina (Al₂O₃) was used as a material of the powder thatsupported Pt thereon. Moreover, the wet pulverizer using thepulverization medium was used for pulverizing the Pt-supported powder.

The Pt-supported powder particles and a binder were mixed with eachother in a weight ratio of 95:5, and were turned into slurry with asolid concentration of approximately 10%. Here, as the binder, the onewas used, which contained the alumina as a main component, and had amean particle diameter that was ½ of that of the Pt-supported powderparticles.

Immediately before drying the slurry obtained by mixing the Pt-supportedpowder particles and the binder with each other, degree of dispersion ofthe slurry was set at 91%. Such setting of the degree of dispersion wasperformed in such a manner that the ultrasonic dispersing device wasarranged immediately before the rapid dryer, the high dispersion outputthereof was adjusted so as to obtain the target degree of dispersion,and the slurry was supplied to the rapid dryer within 1 minute after thehigh dispersion treatment. Here, for the measurement for calculating thedegree of dispersion, the grain size distribution measuring apparatus ofthe dynamic light scattering mode was used, and the degree of dispersionwas calculated by the above-mentioned Expression (1).

A process from drying of the slurry subjected to the high dispersiontreatment to heating treatment thereof is the same as that of Example 1.In such a way, Pt-supported powder was obtained (powder M).

A process from preparation of slurry (slurry M) containing the powder Mthrough coating, drying and baking thereof to formation of a catalyst(catalyst M) is the same as that of Example 2.

A durability test was performed for this catalyst M. In the durabilitytest, the catalyst M was mounted on the exhaust system of the gasolineengine with a displacement of 3500 cc, an inlet temperature of thecatalyst was set at 900° C., and the gasoline engine was operated for 30hours.

With regard to the catalyst M after being subjected to the durabilitytest, powder was sampled from a catalyst coating layer thereof, and acrystallite diameter thereof was calculated from a full width at halfmaximum of the XRD reflection peak of Pt (111). At this time, thecrystallite diameter was 42 [nm].

Comparative Example 9

Comparative example 9 is an example in which the mean particle diameterof the noble metal-supported powder particles is small.

Pt-supported powder was pulverized so that a mean particle diameterthereof could become 50 [nm], and was turned into Pt-supported powderparticles. Alumina (Al₂O₃) was used as a material of the powder thatsupported Pt thereon. Moreover, the wet pulverizer using thepulverization medium was used for pulverizing the Pt-supported powder.

The Pt-supported powder particles and a binder were mixed with eachother in a weight ratio of 50:50, and were turned into slurry with asolid concentration of approximately 10%. Here, as the binder, the onewas used, which contained the alumina as a main component, and had amean particle diameter that was ½ of that of the Pt-supported powderparticles.

A process from high dispersion treatment of slurry obtained by mixingthe Pt-supported powder particles and the binder with each other toheating treatment thereof is the same as that of Example 1.

In such a way, Pt-supported powder was obtained (powder N).

A pore distribution of the obtained powder was measured, and a volumeratio of the pores effective in the gas diffusion, which were present inthe particle inside, was calculated by the above-mentioned Expression(2). At this time, the volume ratio of the pores effective in the gasdiffusion was 20.9%.

Characteristics related to the examples and the comparative examples,which are mentioned above, are shown in FIGS. 7 to 15.

FIG. 7 is a graph showing a relationship between each drying time of thenoble metal-supported powder per piece of the liquid droplets and eachvolume ratio of the pores thereof effective in the gas diffusion. Asapparent from FIG. 7, in Example 1 and Example 6, in which the dryingtime per piece of the liquid droplets is short, a larger number of thepores effective in the gas diffusion were able to be formed in theinside of the noble meta-supported powder in comparison with Comparativeexample 3 in which the drying time is approximately 150 [sec]. Inparticular, in Example 6 in which the drying time per piece of theliquid droplets was set within 1 second, the volume ratio of the poreseffective in the gas diffusion was able to be set at 22% or more.Moreover, in Example 1 in which the slurry was rapidly dried in thestate of the liquid droplets, the volume ratio of the pores effective inthe gas diffusion was enhanced remarkably to 23.8%.

FIG. 8 is a graph showing a relationship between each volume ratio ofthe pores effective in the gas diffusion in the noble metal-supportedpowder and each HC conversion rate of the exhaust gas purifying catalystafter being subjected to the durability test. As apparent from FIG. 8,in the noble metal-supported powder, the purification performance ishigher as the volume ratio of the pores effective in the gas diffusionis larger. The volume ratio of the pores effective in the gas diffusion,which is necessary to obtain 40% as the HC conversion rate after thedurability test, is 22% or more. Here, 40% as the HC conversion rate isthe target in the present invention. Hence, when consideration is madein combination with the relationship between each drying time and eachvolume ratio of the pores, which is shown in FIG. 7, it is understoodthat, in order to obtain the HC conversion rate after the durabilitytest, which is the target in the present invention, it is necessary torapidly dry the slurry within 1 second.

Moreover, in accordance with comparison between Example 2 and Example 3in FIG. 8, the purification performance was able to be further enhancedin such a manner that the Ce-containing oxide powder particles arecontained in the noble metal-supported powder.

FIG. 9 is a graph showing a relationship between the degree ofdispersion of the slurry and each Pt crystallite diameter of the exhaustgas purifying catalyst after being subjected to the durability test. Asapparent from FIG. 9, the Pt crystallite diameter after the durabilitytest is smaller as the degree of dispersion of the noble metal-supportedpowder particles is higher. This represents that the movement andaggregation/particle growth of the noble metal occurs less often afterthe durability test since the noble metal-supported powder particles arefixed in the state of being spaced apart from one another by the highdispersion treatment immediately before the quick drying. Moreover, inaccordance with comparison between Example 5 and Comparative example 6,in Example 5, the Pt crystallite diameter after the durability test wasable to be remarkably reduced than that of Comparative example 6 in sucha manner that the degree of dispersion was set at 33% or more.

FIG. 10 is a graph showing a relationship between the degree ofdispersion of the slurry and each HC conversion rate of the exhaust gaspurifying catalyst after being subjected to the durability test. Asapparent from FIG. 10, the degree of dispersion of the noblemetal-supported powder particles, which is necessary to obtain 40% asthe HC conversion rate after the durability test, is 33% or more. Here,40% as the HC conversion rate is the target in the present invention.Then, in each of Examples 5 and 4 and Example 2, the HC conversion rateof the exhaust gas purifying catalyst after the durability test was asexcellent as 40% or more.

FIG. 11 is a graph showing a relationship between each Pt crystallitediameter of the exhaust gas purifying catalyst after being subjected tothe durability test and each HC conversion rate of the exhaust gaspurifying catalyst after the durability test. As apparent from FIG. 11,it is seen that the Pt crystallite diameter after the durability test,which is necessary to obtain 40% as the HC conversion rate that is thetarget in the present invention, is 15 [nm] or less.

FIG. 12 is a graph showing a relationship between each particle diameterof the binder mixed with the noble metal-supported powder particles andeach Pt crystallite diameter after the exhaust gas purifying catalyst issubjected to the durability test.

As mentioned above, the Pt crystallite diameter after the durabilitytest, which is necessary to obtain 40% as the HC conversion rate that isthe target in the present invention, is 15 [nm] or less. Meanwhile, fromFIG. 12, it is seen that the binder particle diameter that allows the Ptcrystallite diameter after the durability test to be set at 15 [nm] orless is 1500 [nm] or less. Specifically, it is seen that, desirably, thebinder particle diameter is set at 10 times or less that of the noblemetal-supported powder particles.

FIG. 13 is a graph showing a relationship each binder amount of theslurry and each Pt crystallite diameter after the durability test. Asmentioned above, the Pt crystallite diameter after the durability test,which is necessary to obtain 40% as the HC conversion rate that is thetarget in the present invention, is 15 [nm] or less. Meanwhile,referring to FIG. 13, the binder amount that allows the Pt crystallitediameter after the durability test to be set at 15 [nm] or less is 10%or more. Specifically, it is seen that, desirably, the mixing ratio ofthe noble metal-supported powder particles and the binder be set so thatthe mixing ratio of the noble metal-supported powder particles canbecome 90% or less.

FIG. 14 is a graph obtained by comparison among Example 1, Example 10and Comparative example 9, showing a relationship between each particlediameter of the Pt-supported powder particles and each volume ratio ofthe pores effective in the gas diffusion. As seen from FIG. 14, thenoble metal-supported powder particle diameter, which allows the volumeratio of the pores effective in the gas diffusion to become 22% or morethat is necessary to obtain 40% as the target HC conversion rate afterthe durability test, is 90 [nm] or more.

FIG. 15 is a graph obtained by comparison between Example 1 andComparative example 1, showing a relationship between each particlediameter of the Pt-supported powder particles and each volume ratio ofthe pores effective in the gas diffusion. As seen from FIG. 15, thenoble metal-supported powder particle diameter, which allows the volumeratio of the pores effective in the gas diffusion to become 22% or morethat is necessary to obtain 40% as the target HC conversion rate afterthe durability test, is 550 [nm] or more.

Moreover, also referring to FIG. 8 and FIG. 10, which are mentionedabove, the volume of the pores effective in the gas diffusion issufficiently ensured, and the noble metal-supported powder particles arefixed in the state of being spaced apart from one another, whereby thenoble metal in the inside of the powder can also be used effectively topurify the exhaust gas, and accordingly, an exhaust gas purifyingcatalyst having the target HC conversion rate after the durability testwas obtained.

The entire contents of Japanese Patent Application No. 2007-290743(filed on: Nov. 8, 2007) and Japanese Patent Application No. 2008-283480(filed on: Nov. 4, 2008) are incorporated herein by reference.

The description has been made above of the embodiment to which theinvention made by the inventors is applied; however, the presentinvention is not limited by the description and the drawings, which forma part of the disclosure of the present invention by the embodiment. Itis additionally mentioned here that, specifically, other embodiments,examples, operation technologies and the like, which are made by thoseskilled in the art and the like based on the above-mentioned embodiment,are entirely incorporated in the scope of the present invention.

The invention claimed is:
 1. A method of producing noble metal-supportedpowder, comprising: preparing a mixed slurry by mixing noblemetal-supported powder particles and a binder with each other in aliquid; and dispersing the noble metal-supported powder particles byapplying vibrations to the mixed slurry, and thereafter, spray dryingthe mixed slurry while keeping a state where the noble metal-supportedpowder particles are dispersed.
 2. The method of producing noblemetal-supported powder according to claim 1, wherein the noblemetal-supported powder particles are dispersed by applying ultrasonicvibration to the mixed slurry.
 3. The method of producing noblemetal-supported powder according to claim 1, wherein the mixed slurry isprepared by mixing a noble metal-supported powder particle slurrycontaining the noble metal-supported powder particles with a binderslurry containing the binder.